Regarding reflow of these "slabs" -
Don't worry too much about a "shadow" effect.  In my experience (including
going back to Convection/IR reflow of early BGAs, it is a myth that most of
the reflow is air flowing under the component.  If it were, Convection/IR
would never have worked (and it did/does - just ask Compaq) and you;d have to
forget about flip-chips (what with 0.006" height).
Almost all of the heat reflowing a BGA (or similar area array) is conducted
to the site through the circuit board as well as through the component
itself.  So the real concern here is the mass of the component and the amount
of energy it will take to effect reflow.  Of course the composition of the
PCB substrate will either facilitate or hinder things as well.
Regards and Happy Heating to all,
Phil Zarrow
ITM Consulting
Durham, NH  USA
www.ITM-SMT.com

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################